Several feed materials are more or less sensitive to heating and they undergo readily undesired reactions at elevated temperatures when feeding and processing of said feeds.
Heat-induced thermal side-reactions occurring particularly before the feed is brought into contact with an active catalyst bed can yield side-reaction products, such as polymeric substances, which may form fine particles and larger agglomerates causing fouling of the reactor and pipes and eventually plugging of the active catalyst bed.
Examples of heat-sensitive feeds are feedstocks comprising fatty acids and/or derivatives thereof like palm oil, soybean oil, rapeseed oil and sunflower oil, as well as fats and oils originating from animals and fish. Said starting materials may be used for example in hydrotreating reactions. When a feedstock comprising fatty acids and/or derivatives thereof enters a hydrotreating reactor at elevated temperatures and it is brought into contact with a hydrotreating catalyst bed for effecting hydrotreating reaction to obtain paraffinic hydrocarbons, adverse thermal side-reactions may occur. Examples of such thermal side-reactions include polymerization of double bonds in the fatty acid chains, which is dramatically increased with increased temperature. In said polymerization two fatty acid molecules, which may originate from triglycerides, are coupled together at the double bonds of the molecules and the double bond of the formed molecule is shifted to another position. In the case of conjugated double bonds further polymerization is even more likely to occur.
Free fatty acids may undergo similar dimerization, oligomerization and polymerization reactions. Fatty acids of plant origin typically contain several double bonds, like the polyunsaturated fatty acids (PUFA) of soybean oil. Conjugated double bonds of PUFA undergo readily polymerization reactions and they are therefore very sensitive for excess heating.
Hydrotreating of fatty acids and derivatives of fatty acids is typically carried out by hydrodeoxygenating/hydrogenating at an elevated temperature on a catalyst bed containing a solid metal catalyst. The reactor is usually equipped with a recycling line in order to recycle a part of the formed hydrotreated product back to the reactor feed. In the hydrodeoxygenation and/or hydrogenation of fatty acids and/or derivatives thereof the reaction temperature must typically be raised gradually with ageing of the catalyst, which in turn increases the adverse side-reactions. Heavier molecules formed in the side-reactions significantly decrease the quality of the hydrotreated product and also lead to problems if the hydrotreated product is subsequently subjected to an isomerization reaction step.
For effecting the desired conversion in the hydrotreatment process the temperature of the feed must be raised to the level required by the hydrodeoxygenation and hydrogenation reactions prior to feeding it to the reactor and to the active catalyst bed. Typically residence times in these hydrotreatment processes are around 1 min.
In practice, the temperature of the feed has been adjusted to the desired range by mixing a fresh feed with hot product recycle before introducing the mixture to the reactor. At the same time the fresh feed is diluted when it comes into contact with the product recycle. This dilution recycling facilitates an economical way of elevating the process temperature and adjusting the concentration of the substances contained in the feedstock to a suitable level in the feed.
Publication US20070010682 describes a method for the manufacture of diesel range hydrocarbons wherein a feedstock comprising triglycerides and/or fatty acids is hydrotreated and isomerized and the fresh feed is diluted with a recycled hydrotreated product stream.
A process for continuous hydrogenation of triglyceride containing raw materials in a fixed bed reactor system having several catalyst beds arranged in series is disclosed in EP 2226375. A vegetable oil raw material feed, hydrogen containing gas and diluting agent are passed to each catalyst bed. The weight ratio of the diluting agent to the raw material feed at the entrance of each catalyst bed should be essentially the same at the entrance of all catalyst beds and should not exceed 4:1. This process provides low downstream hydraulic load and allows the use of existing units without considerable revamp.
Therefore it is the object of the present invention to provide a method and arrangement for feeding heat-sensitive materials to fixed-bed reactors comprising recycling whereby the residence time of the heat-sensitive material and adverse side-reactions can be substantially decreased.